1. Field of the Invention
The present invention relates to telecommunications systems and, in particular, to a differential dibit phase shift keying (DBSK) data recovery circuit which utilizes a convolution technique to generate decoded data outputs.
2. Discussion of the Prior Art
The basic function of a communications system is to transmit information from a source to a destination as fast and as accurately as possible. The source and destination are physically separated from one another and are connected by a communications channel.
There are two types of information sources: analog information sources and discrete information sources. Analog sources, such as a microphone, produce a continuous amplitude signal. Discrete sources, such as a digital computer, generate a signal consisting of a sequence of characters. Analog signals can be converted to discrete signals by utilizing sampling and quantizing techniques.
Communications channels which are designed to handle voice transmission (i.e., the telephone network) have characteristics which make it difficult for them to transmit binary digits or bit streams. To permit the transmission of digital data over a voice channel, it is necessary to utilize the digital data to modulate a carrier waveform having a frequency which is compatible with the voice channel.
The equipment which performs the required modulation is generally referred to as "modem". The term "modem" is an acronym for MOdulator-DEModulator since the equipment typically includes the capability not only to modulate transmitted signals but also to demodulate received signals.
Generally speaking, as stated above, a modulator receives a serial digital data bit stream from an information source and converts the bit stream to a waveform suitable for transmission over the communication channel. In addition to matching the frequency spectrum of the transmitted signal with the characteristics of the communication channel, the modulator also serves to minimize the effect of signal distortion caused by noise and by the non-ideal nature of the communications channel.
At the receiver end of the communications system, the transmitted signal is demodulated; that is, the transmitted data is separated or recovered from the modulated carrier waveform.
For communication systems that use higher data rates, i.e., 1200 baud or greater, various forms of phase shift keying (PSK) modulation are utilized.
A 2-phase PSK modulation system uses one phase of the carrier frequency for one binary state and the other phase for the other binary state. The two phases are 180.degree. apart and are detected by a synchronous recovery device using a reference signal at the receiver which is of known phase with respect to the incoming signal.
A differentially coherent PSK (DPSK) modulation scheme eliminates the need for a reference signal at the receiver. In a DPSK scheme, the phase reference for demodulation is derived from the phase of the carrier waveform during the preceding signaling interval. The receiver recovers the digital information based on the phase differential.
In one type of four-phase DPSK signalling scheme, the binary data to be transmitted is grouped into blocks of two bits called dibits. The resulting four possible dibit combinations, i.e. 00, 01, 10 and 11, differentially phase modulate the carrier waveform. The receiver produces two outputs which are proportional to the phase difference in the transmitted signals. The input bits are uniquely recovered based on the characteristics of the two outputs. The recovered dibits are then converted to a serial binary output.
Generally speaking, convolution is a mathematical operation which is used to "smooth" or "average" functions of the same variable.
Conventional DBPSK recovery circuits require A/D converters. In addition, these digital data processing techniques are expensive because of their high power consumption and very high system toggling frequencies which result in heavy investments in RFI shieldings.